This invention relates to extruding and more particularly to a new and improved apparatus for annealing extruded thermoplastic materials on an in-line production facility.
The extrusion of low stress or stress-free hollow profiles is necessarily done at low linear rates and low stock temperatures in order to minimize differences in the cooling rate between the internal and the external wall surfaces, otherwise stresses are introduced into the materials. In order to develop optimum properties in certain thermoplastic materials, it is desirable to extrudate at high extrusion rates and at high stock temperatures, however, the more severe cooling conditions needed inevitably lead to the formation of circumferential stresses during the sizing and cooling process. When such hollow profiles are cut longitudinally to provide separate longitudinal sections, the built-in stress will cause a deformation of shape in the profile. In a similar manner, when the initial profiles are subjected to temperatures close to their heat distortion temperatures, the stresses are relaxed, resulting in a deformation of the shape of the profile.
The present invention provides the means and the ability to significantly increase the extrusion rate at high temperatures by providing a new and improved means for annealing the shaped profile. The present invention is particularly useful on hollow extruded materials wherein the end product requires that the profile be cut longitudinally as heretofore any residual stresses that were built in become quite apparent upon cutting the product longitudinally. Such apparatus is useful in the processing of thermoplastic house siding in the form of a hollow rectangular shaped profile permitting easier handling, sizing and processing. This permits the extrudate to be processed at higher temperatures and higher speeds to provide better mechanical properties and excellent weathering characteristics. With proper annealing, the siding can be produced faster and more economically since the hollow shaped rectangular profile can be cut longitudinally to double production while producing stress-free siding. Such annealing means is adaptable to other shapes as well.